Recently, systems called “content distribution systems” have been developed. Each of the “content distribution systems” makes it possible to distribute digital content (simply referred to as “content” hereinafter) such as music, video, games, or the like from a server device to a terminal device through communication such as the Internet and digital broadcast, and use the content in the terminal device. In a general content distribution system, a digital rights management technique is used in order to protect the copyright of the content and prevent a malicious user from using the content in an unauthorized manner. More specifically, this “digital rights management technique” is a technique for securely controlling the use of the content by using an encryption technique or the like.
For example, paid broadcasting employs a scramble control method according to which a video signal and an audio signal are scrambled and transmitted, and only the terminals having a viewing right descramble the video and audio signals to enable viewing of the content.
Details of the above-mentioned conventional scramble control method are disclosed in Non-patent Reference 1 and Non-patent Reference 2.
The conventional scramble control method utilizes two types of information items called individual information and program information (referred to as “content-related information” hereinafter in the present invention). The individual information is information indicating a subscription contract made for each of receivers, and includes a work key which is required to decrypt the details of the contract, the valid periods of the contracts, and content-related information items to be transmitted later on. The content-related information is information to be transmitted in parallel with the scrambled video and audio signals and the like, and includes a scramble key which is required to descramble these signals, the current time indicating the moment, and the details of the program.
The terminal device which receives the content receives the individual information addressed to the terminal itself and holds the details as the contract information prior to the reception of the content-related information. In the viewing of the program, the terminal device receives content-related information together with the video and audio signals, checks the current time and the valid period of the contract which are included in the content-related information, and checks whether or not the contract has expired. In the case where the content has expired, the terminal device judges to be “No viewing right”. In the case where the content has not expired, the terminal device checks the presence/absence of a viewing right by checking the program information and the contract information. Here, only when the judgment shows that “viewing right exists”, the terminal device extracts the scramble key and descrambles the video and audio signals and the like.
In this way, the conventional scramble control method makes a judgment on the valid period of a contract based on the current time which is transmitted together with the content.
[Non-patent Reference 1]
    Request No. 17 for recommendation of Telecommunications Technology Council, Showa 63 (1988), Eisei housou ni yoru television housou ni okeru yuryo housiki ni kansuru gijutsu teki joken (Technical conditions for paid scheme in satellite television broadcasting)[Non-patent Reference 2]    Request No. 74 for partial recommendation of Telecommunications Technology Council, Heisei 7 (1995), Digital housou houshiki ni kakaru gijutsu teki joken (Technical conditions for digital broadcasting scheme)